1. Field of the Invention
The present invention relates to measurement of changes in length or distance. More specifically, this invention is directed to apparatus for providing an electrical signal commensurate with variations in spacing between objects and particularly to devices for sensing changes in the distance between moving objects. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited for use in monitoring the spacing between moving objects such as, for example, railway cars. It is important for the engineer of a train, particularly a freight train, to know whether the couplings between cars of his train are compressed or extended. As is well known, in order to facilitate starting, the couplers between railway cars are designed to have a degree of slack. Thus, during motion the cars are free to move relative to one another and, unless appropriate measures such as braking or speed variation are taken by the engineer, accelerations and bumping can occur which may damage cargo.
In the prior art most devices capable of measuring changes in length or distance are suitable only for laboratory use such as, for example, in the testing of materials for deformation under strain. Such laboratory instruments are not suitable for use in the field because of their size and/or fragility. Additionally, such laboratory instruments can not be readily adjusted to work over differing ranges of measurement and generally are not suitable for the measuring of distance changes of comparatively large magnitude. As a further deficiency of the prior art laboratory-type instruments, the considerble expense of the apparatus weighs against its being left unattended; i.e., connected to telemetry transmitters and/or recording apparatus.
Comparatively inexpensive length or distance change measurement apparatus of the extensiometer or "string pull" type have been proposed. In such apparatus the unwinding of a string or wire causes the wiper arm of a potentiometer to be driven and the resultant resistance change is commensurate with changes in length or distance. The movements of the potentiometer wiper arm may thus be employed to generate an electrical output signal which can be monitored and/or recorded. Extensiometer devices of the "string pull" type are characterized by a number of serious operational problems. Thus, by way of example, under rapid distance changes the prior art extensiometer devices tend to bind as a result of the string escaping off the storage reel and jamming the mechanism. Such binding usually results in breaking of the string. Further, in order to achieve linearity over a wide operating range, the potentiometer employed must possess a high degree of uniformity of resistance. This requirement, in turn, dictates use of an extremely linear multi-turn potentiometer; such potentiometers comprising a single strand resistance wire helically wound on a bobbin. Potentiometers employing a single strand resistance wire, as is well known in the art, are prone to "jitter" failure resulting from the wiper arm wearing out a section of the resistance element along which it moves. Nevertheless, prior art "string pull" extensiometers have accepted the fragility incident to use of a single strand potentiometer in order to achieve accuracy and linearity over their design range of distance changes.